1. Field of the Invention.
Current processes for the fractionation of whey proteins that rely on the addition of chemical agents result in contamination of the proteins, requiring additional steps to remove the contaminants. These chemicals can have an adverse effect on protein functional behavior and its ultimate use in food products. This invention relates to a process for fractionating whey proteins with high pressure CO.sub.2 or supercritical CO.sub.2.
2. Description of the Related Art.
Sweet whey, a watery by-product of the cheese manufacturing process, comprises almost 7% solids. Proteins account for about 10-12% of the solids content, the rest being mainly lactose, minerals, milkfat and lactic acid at 74%, 8%, 3% and 3%, respectively (Morr, D. V. 1989. In: Developments in Dairy Chemistry--4. Fox, P. F. ed., Elsevier Applied Science, New York.) With the advent of ultrafiltration, whey protein concentrates (WPC) ranging from 35 to 85% protein content have been obtained. Depending on protein content, these concentrates have properties such as good solubility, emulsification, water binding, foam formation and gelation, and because of their nutritional properties, they are useful for protein fortification. The functionality generally improves as the protein content increases. For example, WPC with lower protein content are best for protein fortification while those of higher protein content impart texture and gelling properties.
The proteins, .alpha.-lactalbumin (.alpha.-La), .beta.-lactoglobulin (.beta.-Lg), bovine serum albumin (BSA), and heavy and light chain immunoglobulins (Ig) comprise the principal whey proteins. Whey also contains minor proteins such as lactoferrins (Lf), proteose peptide components, glycomacropeptides and low molecular weight enzymatic degradation products of the caseins (DeWit, J. N. 1989. In: Developments in Dairy Chemistry--4. Fox, P. F., ed. Elsevier Applied Science, NY). The properties of the .beta.-Lg fraction are desirable in binding and gelation, and the .alpha.-La fraction may find use in humanized infant formulas (Heine et al., WO 92/03468, Mar. 5, 1992).
Many methods have been reported for fractionation of the whey proteins into .alpha.-La and .beta.-Lg enriched fractions. These methods generally rely on gentle heat treatment and pH adjustment (Pearce, R. J. 1983. Australian J. Dairy Technol. vol. 38, p. 144; Pearce, R. J. 1987. Australian J. Dairy Technol. vol. 212, p. 150; Pearce et al. 1991. Food Res. Quarterly. vol. 51, p. 137), addition of salts (Aschaffenburg and Drewry. 1957. Biochem J. vol. 65, p. 273; Kaneko et al. 1985. J. Food Sci. vol. 50, p. 1531; Kuwata et al. 1985. J. Food Sci. vol. 50, p. 605; Al-Mashikh and Nakai. 1987. J. Food Sci. vol. 52, p. 1237; Maillart and Ribadeau-Dumas. 1988. J. Food Sci. vol. 53, p. 743; Mate and Krochta. 1994. J. Food Sci. vol. 59, pp. 1111-1113), separation using a PEG/potassium phosphate aqueous two-phase system (Chen, J. 1992. J. Ferment. Bioeng. vol. 73, p. 140), extraction with organic solvents (Heine, supra), microfiltration (Uchida, T. EP 0 556 083, Jan. 28, 1993), anion exchange (Venter, B. G. 1984. S. Afr. J. Dairy Technol. vol. 16, p. 79; Thibault. P. A., U.S. Pat. No. 5,077,067, 1991), cation exchange (Ohtomo et al. 1988. Japanese Soc. Food Sci. Technol. vol. 35, pp. 755-762), ultrafiltration (Roger et al. U.S. Pat. No. 4,711,953, 1987), various chromatographic methods (Biscans et al. 1985. Entropie N.degree.. vol. 21, p. 17; Chiancone and Gattoni. 1993. Biotechnol. Appl. Biochem. vol. 18, pp. 1-8; Carrere et al. 1994. Trans IchemE. vol. 72, p. 216), or techniques using ultrafiltration and electrodialysis (Amundson and Watanawanichakorn. 1982. J. Food Proc. and Preserv. vol. 6, p. 55; Slack et al. 1986. J. Food Proc. and Preserv. vol. 10, p. 19). Preliminary experiments utilizing carbon dioxide were also reported by Tomasula et al. (1995. International Chemical Congress of Pacific Basin Societies, Honolulu, Hi.).